gh-91153: Fix bytearray holding a reference to its internal buffer when calling into potentially mutating __index__ methods

[bast0006]

bytearray's __setitem__ implementation currently grabs a reference to its internal buffer before calling _getbyvalue to determine the index that needs assignment. _getbyvalue can call into arbitrary python code via __index__ dunders, which could alter the internal buffer and leave said reference dangling.

A prior fix for this issue ensures that bounds checking occurs after _getbyvalue is called. However, python code is capable of resizing the bytearray, resulting in limited but still broken behavior.

This patch ensures that the reference to the internal buffer is fetched only after _getbyvalue is called to prevent it from being held while any python code is run.

• Issue: UAF when writing to a bytearray with an element implementing __index__ with side-effects #91153

[python-cla-bot]

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[bast0006]

I've signed the CLA. Let me know if a NEWS entry is required. I'm not sure it is, especially since there was already a previous change that this is a fix to.